Filter collection on associated field ObjectId - node.js

Question

SEE EDIT AT BOTTOM OF QUESTION.
I have a Node.js Express web application using MongoDB and Mongoose with collections for articles and comments. They have a one-to-many association where one article can have many comments.
The mongoose model schema is as follows:
// models/article
const mongoose = require('mongoose');
const articleSchema = new mongoose.Schema({
title: { type: String },
content: { type: String },
}, {timestamps: true});
module.exports = mongoose.model('Article', articleSchema);
and
// models/comment.js
const mongoose = require('mongoose');
const commentSchema = new mongoose.Schema({
content: { type: String },
article: { type: mongoose.Schema.Types.ObjectId, ref: 'Article' },
}, {timestamps: true});
module.exports = mongoose.model('Comment', commentSchema);
I have a route with a parameter for the article id
// routes.js
router.get('/articles/:articleId/comments', commentsController.list);
And a controller with a callback function to query the database and return the comments with the given article id. It uses the mongoose find() method filtering on the article id taken from the route parameter.
// controllers/commentsController.js
exports.list = (req, res, next) => {
Comment.find({ article: req.params.articleId })
.exec((err, comments) => {
res.render('comments/list', { title: 'Comments', comments: comments });
});
};
But this turns up no results. Just experimenting I can see that the req.params.articleId is a string and any comment.article is an object so they match with a loose comparison == but not a strict comparison === unless I convert comment.article.toString(). Anyway, what is the proper way to do such a query. All my attempts have failed.
EDIT: I found the problem. The code above is as it should be. The issue must be related to how I seeded the DB which I did directly in MongoDB. I deleted all those records and just added them from the application and it works with the code above.

Answer 1

One way to approach this is to add the comments to your article model.
const mongoose = require('mongoose');
const articleSchema = new mongoose.Schema({
title: { type: String },
content: { type: String },
comments: [
{
type: mongoose.Schema.Types.ObjectId,
ref: 'Comment'
}
]
}, {timestamps: true});
articleSchema.set('toJSON', {
transform: (document, returnedObject) => {
const article = returnedObject
article.id = article._id.toString()
delete article._id
}
})
module.exports = mongoose.model('Article', articleSchema);
Then get the comments in one of these ways:
const router = require('express').Router()
const Article = require('../models/article')
const Comment = require('../models/comment')
// article with comments
router.get('/:id', async (request, response, next) => {
try {
const article = await Article.findById(request.params.id)
.populate(
'comments', {
content: 1
}
)
response.json(article.toJSON())
} catch (err) {
console.log(err)
}
})
// list of comments belonging to an article
router.get('/:id/comments', async (request, response, next) => {
try {
const article = await Article.findById(request.params.id)
if (!article) {
response.status(404).json({ error: 'invalid request' })
}
const comments = await Comment.find({ article: request.params.id })
.populate(
'article', {
title: 1
}
)
response.json(comments.map(comment => comment.toJSON()))
} catch (err) {
console.log(err)
}
})
module.exports = router